The electromagnet includes a coil assembly consisting of at least one circular cylindrical field coil and at least one correction coil concentric with the field coil. The coil assembly is enclosed in, and simultaneously supported by, a stable cylindrical shell made of iron whose influence on the homogeneity of the magnetic field, which is produced by the coil system in an interior space defined by it and is suitable and accessible to accommodate an organic body to be examined, is compensated by the particular dimensioning of the field and correction coils, as it is described in U.S. patent application Ser. No. 386,981, filed June 10, 1982 now U.S. Pat. No. 4,490,675, issued Dec. 25, 1984.
The electromagnet disclosed in the aforementioned patent application is constructed essentially in the form of a long solenoid either of the single piece type or made up of a plurality of composite parts. Secured to the ends of the solenoid are correction or error coils. It is thus possible to produce coil systems which have an especially small diameter while being of considerable length and which are therefore particularly well suited to accommodate a human body for examination. Such coil systems are very similar to the coil systems described by Garrett in the Journal of Applied Physics, Vol. 42, No. 8 (July 1969), pages 3171-3179. However, the values given by Garrett for these coil systems must be modified to compensate for the influence exerted by the ferromagnetic cylindrical shell.